Carbon fiber felt is excellent in heat resistance to a high temparature, thermal insulating properties and the like. Accordingly, such felt is used as a thermal insulator in a high-temperature furnace such as a ceramic sintering furnace, a vacuum furnace for evaporation deposition of metal, a furnace for growing semiconducting single crystals, or the like.
On the other hand, the inventors have reported in Collection of Outlines of Lectures, 1143-1148P, of First Japan International SAMPE Symposium & Exhibition (Nov. 30, 1989) that the dependency of thermal conductivity on temperature varies with the bulk density of carbon fiber felt. More specifically, as shown in FIG. 1, the thermal conductivity .lambda. serving as an index of thermal insulating properties in a high-temperature furnace closely relates to the bulk density .rho. of carbon fiber felt. In a high-temperature zone, the thermal conductivity .lambda. generally becomes smaller as the bulk density .rho. is greater. In a low-temperature zone, the thermal conductivity .lambda. generally becomes smaller as the bulk density .rho. is smaller. Further, the thermal insulating properties become greater as the carbon fiber felt is thicker.
Since the carbon fiber felt is excellent in electric conductivity, it has been proposed to use the carbon fiber felt as a material for the electrodes of a secondary battery of the Na-S type or the like. Since the electrode material is required to have a number of electric active sites, a predetermined repulsion force or the like, it has been considered that carbon fiber felt having bulk density of 0.1 g/cm.sup.3 or more is desired as such an electrode material.
In view of the foregoing, (1) the Japanese Patent Publication No. 35930/1975 proposes a method of manufacturing a molded thermal insulator comprising the steps of:
impregnating carbon fiber felt with resin which can be carbonized or graphitized;
winding the resin-impregnated felt on a mandrel;
mounting a thin steel sheet on the outer circumference of the felt thus wound on the mandrel;
fastening the wound felt and sheet with a belt or the like, causing the felt to be compressed, thereby to produce a hollow cylindrical molded article having desired thickness and bulk density; and
carbonizing or graphitizing the molded article.
The Japanese Utility Model Publication No. 29129/1983 proposes a multi-layer molded thermal insulator for a vacuum furnace, comprising (i) permeable carbon fiber felt sheets formed through the steps of impregnation with a resin solution, compression and carbonization, and (ii) graphite sheets with a thickness of 1 mm or less having sealing properties, the felt sheets and the graphite sheets being alternately laminated through adhesives.
To increase the bulk density of the molded thermal insulator above-mentioned, there are required a variety of steps, i.e., resin impregnation, compression-molding, drying-setting, and calcination. In the resin impregnation step, it is required to use a viscous resin solution which decreases the workability. Further, the resin-impregnated felt is subjected to compression-molding and drying-setting. This not only takes a lot of time for molding, but also requires treatment with an organic solvent. Accordingly, the workability and the productivity are lowered.
It is difficult to uniformly impregnate the carbon fiber felt with resin, and the felt is fastened, at the compression-molding step, with a band or the like. Accordingly, the resultant molded thermal insulator lacks uniformity. The molded thermal insulator is integrated with carbonized resin and is hard. Accordingly, the molded thermal insulator lacks resiliency and cushioning properties. This causes the thermal insulator to be easily broken at the time of processing or attachment thereof in a furnace. Accordingly, when attaching, to a furnace, a sheet-like thermal insulator or a thermal insulator having a curved section with both end surfaces thereof bonded to each other, it is difficult to align the end surfaces to be bonded and to closely bond the end surfaces to each other. This produces gaps between the bonded end surfaces to lower the thermal insulating properties.
The molded thermal insulator obtained through a compression-molding step presents the same bulk density in the thickness direction thereof. This does not provide sufficient thermal insulating properties in a high- or low-temperature zone.
In this molded thermal insulator, the restoring force required as a heat-resisting cushioning material is small, and the durability is therefore not sufficient.
Since the felt is calcined after resin-impregnation, the molded thermal insulator is apt to be easily warped. Further, while being machined or used, the thermal insulator generates a great amount of powder due to impregnated resin. This involves the likelihood that the powder thus produced contaminates workpieces to be heated in a high-temperature furnace.
On the other hand, (2) as to mechanically bonded carbon fiber felt without impregnation with resin, the bulk density is small. Accordingly, the thermal insulating properties at a high temperature are small. Therefore, such felt is not suitable as a thermal insulator for a high-temperature furnace. To ensure the thermal insulating properties at the time of heat treatment at a high temperature, it is required to attach a plurality of felt pieces to a large-size high-temperature furnace or the like. This presents the problem that the felt attachment is troublesome.
Further, the felt itself has small mechanical strength and lacks the shape holding properties. This makes it difficult to handle the felt. For example, felt made of, as a starting material, phenol resin-type fibers which can be carbonized, has small bulk density and small mechanical strength when the felt has a thickness of 3 mm. Accordingly, a foundation cloth is required. As to carbon fiber felt obtained by carbonizing felt having a thickness of about 5 to about 7 mm, the bulk density at the time when no load is applied, is generally as small as 0.1 g/cm.sup.3, and the thickness is also small. Thus, the thermal insulating properties are lowered. If such felt is graphitized, the bulk density is further reduced, thereby to lower the thermal insulating properties. As to carbon fiber felt obtained by graphitizing felt having a thickness of 10 mm or more, the bulk density is generally lowered to about 0.08 g/cm.sup.3. This is presumably caused by great decrease in weight and great reaction heat at the time of carbonization or graphitization.
When rayon or polyacrylonitrile fibers which are a carbon fiber material, are needled, the bulk density of felt before the fibers are carbonized, is increased. However, at the time of carbonization and graphitization, the weight is considerably decreased and the bulk density is considerably decreased. The resultant carbon fiber felt presents small mechanical strength, causing the felt to be easily broken. Thus, the durability is insufficient.